Synchrotron radiation in situ study on liquid-solid thermomigration in Cu/Sn/Ni solder joint

Synchrotron radiation real-time imaging technology was conducted to in situ investigate the liquid-solid thermomigration behavior in Cu/Sn/Ni solder joint. Asymmetrical growth of the intermetallic compounds (IMCs) at the two ends (interfaces) was clearly observed in the Cu/Sn/Ni solder joint during reflow on a hot plate. The thickness of the IMC layer at the cold end increased linearly with reflow time, while that at the hot end changed little, resulting in increasing difference in IMC thickness between the two ends with reflow time. Due to the thermomigration of Ni atoms toward the cold end under temperature gradient, the (Cu,Ni)6Sn5 IMCs grew quite fast at the cold end. The changes in the morphology of the (Cu,Ni)6Sn5 grains at cold end associated with the increasing Ni content. Cu atoms from the cold end could migrated across the liquid solder layer to the hot end against temperature gradient, resulting in the formation of a smooth (Cu,Ni)6Sn5 layer at the hot end.